This invention relates to improvements in a rotor for use in a rotary electric machine, particularly to improvements in a rotor having winding-retaining rings mounted adjacent to the axial ends of a rotor core, respectively.
In general, the rotor in a rotary electric machine is arranged such that a rotor core is fitted on a rotary shaft, and insulated windings are wound around the core. With such an arrangement, the windings are retained in position by means of wedges and retaining rings. More particularly, part of windings which extends in grooves in the core is retained relative to the core by means of wedges forcedly fitted in the grooves, while axial end portions of windings, which are protruded from the axial ends of the core respectively, are retained in position by means of cylindrical retaining rings respectively which surround the peripheries of the protruded end portions of the windings respectively and are secured to a rotary shaft.
Thus, the retaining rings retain in position the end portions of the windings respectively so as to prevent the end portions from being radially outwardly moved due to the centrifugal force during the rotation of the rotor. However, with the rotor designed for a high peripheral speed, there results an extremely large centrifugal force in the protruded end portions of windings, in addition to the centrifugal force acting on the retaining rings. As a result, extremely severe hoop stress acts on each retaining ring. For this reason, each retaining ring should afford a tough construction and be made of a material having high strength.
Meanwhile, limitations are imposed on a material of the retaining ring, because of a need to use a non-magnetic steel, the aforesaid limitations arising from the consideration on a magnetic influence of the retaining rings during their operation as well as from the availability of a material.
In the practical application, however, considerations are given to the shape of each retaining ring itself and the arrangement of each retaining ring for minimizing stress to be created thereon.
With a high-capacity electric machine or a rotor designed for a high peripheral speed, which have to be subjected to severe stress, the diameter and thickness of each retaining ring are somewhat increased, while sacrificing compactness of an electric machine.
Meanwhile, heat power plants are often constructed in a site or on a bridge constructed in the sea. In such a case, it happens that cooling gas for use in a rotary electric machine necessarily contains salt or moisture to some extent.
The cooling gas containing such impurities exerts an adverse influence on the strength of each retaining ring for the reason which will be described hereinafter. Test results reveal that such chemical attacks can not be overcome only by resorting to an increase in wall thickness of each retaining ring.
More specifically, inclusion of salt and moisture in cooling gas leads to a most thorny problem, i.e., corrosion in members of a rotary electric machine which are made of steels in majority.
Many attempts have been proposed to prevent the members of a rotary electric machine from being subjected to corrosion, for instance, the application of a corrosion-preventive coating. Meanwhile, it is true that, even if corrosion takes place, there remains only a reduction in thickness of steel members, providing no serious problem for the operation of an electric machine. However, this can not be neglected in the aforesaid retaining ring of a rotor. During the tests given by the inventors, considerations have been given to the facts that a centrifugal force is produced on a rotor due to its rotation and cooling gas is used for cooling the protruded end portions of the windings housed respectively in chambers defined by the inner peripheral surfaces of the retaining rings and the periphery of the shaft, and that salt and moisture are collected on the inner peripheral surfaces of the retaining rings under the action of a centrifugal force. Thus, a combination of stresses arising from the aforesaid centrifugal force with the influence of the aforesaid salt and moisture leads to a stress-corrosion-cracking problem in the inner peripheral surfaces of the retaining rings. It was found that due to the aforesaid cracking, the retaining rings would cause a premature failure during their service, because of a sudden build-up of stresses.
As has been described, it would be no serious problem, if retaining rings cause simple corrosion, i.e., reduction in wall thickness, like the other parts such as wedges or a current-collecting ring, which would cause no sudden build-up of stresses. The stress-corrosion cracking as caused in the retaining rings is considered to be attributed to the fact that when the cooling gas is introduced through inlet openings in end rings into the aforesaid chambers, then the cooling gas is forced toward inner peripheral surfaces of the retaining rings due to a centrifugal force created by the rotation of a rotor, so that the cooling gas containing moisture and particularly salt clings to the inner peripheral surfaces of the retaining rings, with the result that moisture is evaporated due to heat in the retaining rings (heat transmitted from windings), thereby leaving salt thereon. A repeated cycle of such a phenomena leads to an increase in concentration of salt, whereby stress-corrosion cracking tends to take place on a portion of the inner periphery of each retaining ring adjacent to the inlet openings for the cooling gas most frequently, because the salt is apt to most likely cling to the portion of the inner periphery of each retaining ring.